The publication FR-A 2,096,133 describes a seat in which the squab is equipped with a cushion containing a resiliently compressible porous mass connected to a reduced-pressure source. This source adjusts the thickness of the cushion until its rigidity is sufficient to resist the compression caused by the load communicated by the user of the seat. It has been found, however, that such a design of the squab is poorly suited to the requirements of lateral retention of the trunk and of sacrolumbar lordosis during the changes in posture of the user of the seat.
The publication EP-A 0,113,613 describes a seat including a partitioned PVC cover containing a layer of particles such as polystyrene balls. This cover is connected by means of a pipe to a vacuum pump. Once suction has been established, the walls of the cover compress the polystyrene balls without any substantial variation in volume, creating a coherence between the balls which hardens the layer in the position required by the sitting position of the user of the seat.
The bursting of the balls following external vibrations and friction communicated to the cover results in a total loss of effectiveness of the lining element. Furthermore, it proves difficult to distribute the balls satisfactorily when the cover is situated in a non-horizontal plane.
The publication EP-A-267,640 describes a cover for surgical use which is capable of preserving a shape by the vacuum effect and in which balls are held in place by adhesive bonding on layers of non-woven fabric or in a plastic network produced by injection-molding.
The publication DE-GM 7,617,960 furthermore discloses a backrest consisting of pneumatic sleeves. Pumps, each equipped with an outlet valve, make it possible to adjust individually the internal pressure of each sleeve and to adapt the backrest to the requirements of the user. It is difficult to modify the support effect while the vehicle is moving, in particular for the driver, and it is possible only if the operating member has been designed in order to be used with just one hand.